This invention relates to a glass ceramic laminate which is for use in producing a substrate, such as a wiring board or a circuit board, of a laminate device and which can be fired at a low temperature not higher than 1000xc2x0 C. and to a sintered glass ceramic product using the glass ceramic laminate.
A substrate, such as a wiring board or a circuit board, of the type has a structure in which an internal conductor is held by a dielectric material. As the internal conductor, use is preferably made of silver or copper low in conductor loss. As the dielectric material, use has widely been made of alumina ceramic. However, alumina ceramic has a firing temperature as high as 1600xc2x0 C. Therefore, the internal conductor which can be co-fired is restricted to a high-melting-point metal such as molybdenum and tungsten. Such high-melting-point metal has a high conductor resistance, which results in an increase in transmission loss.
Under the circumstances, a glass ceramic material which can be fired at a lower temperature is developed as the dielectric material and put into practical use. The glass ceramic material of the type includes glass powder and ceramic powder and can be fired at a temperature not higher than 1000xc2x0 C. Therefore, the glass ceramic material is advantageous in that silver or copper low in conductor loss can be used as the internal conductor.
However, the glass ceramic material is generally low in bending strength as compared with alumina ceramic. Therefore, if the glass ceramic material is used in a personal digital assistant (PDA) or the like which is often subjected to mechanical shock, the substrate is easily broken and the reliability is low in a fall test.
It is therefore an object of the present invention to provide a glass ceramic laminate which can be fired at a temperature not higher than 1000xc2x0 C. and which becomes relatively high in bending strength after fired.
It is another object of the present invention to provide a sintered glass ceramic product which is made from the glass ceramic laminate of the type described.
Other object of the present invention will become clear as the description proceeds.
According to one aspect of this invention, there is provided a glass ceramic laminate comprising at least three glass ceramic layers stacked and press-bonded to be substantially symmetrical in a stacking direction, each of outermost ones of the glass ceramic layers and an inner layer adjacent thereto being formed by glass ceramic materials which, when fired, have crystal phases different from each other, each of the outermost layers having a first thermal expansion coefficient xcex11, the inner layer having a second thermal expansion coefficient xcex12, the relationship between the first and the second thermal expansion coefficients xcex11 and xcex12 being given by 0 less than xcex12xe2x88x92xcex11 less than 5 ppm.
According to another aspect of this invention, there is provided a sintered glass ceramic product comprising at least three layers formed by glass ceramic materials which are stacked to be substantially symmetrical in a stacking direction and integrally sintered, each of outermost ones of the layers and an inner layer adjacent thereto having crystal phases different from each other, each of the outermost layers having a first thermal expansion coefficient xcex11, the inner layer having a second thermal expansion coefficient xcex12, the relationship between the first and the second thermal expansion coefficients xcex11 and xcex12 being given by 0 less than xcex12xe2x88x92xcex11 less than 5 ppm.